Fullerene, carbon nanotubes, graphene, graphite, and the like are low-dimensional nano-materials composed of carbon atoms. That is, carbon atoms arranged in a hexagonal shape may form zero-dimensional fullerene formed of a ball, may form carbon nanotubes one-dimensionally rolled, may form graphene of a two-dimensional monolayer, and may form graphite three-dimensionally stacked.
In particular, graphene has very stable and excellent electrical, mechanical, and chemical characteristics and is a very excellent conductive material in which electrons can move about 100 times faster than in silicon and current flows about 100 times more than in copper. This was demonstrated through experiments in 2004 when a method of separating graphene from graphite was found. Since then, a great deal of research on this matter has been carried out.
Graphene is made of pure carbons which are relatively light atoms, and, thus, it is very easy to process graphene in a one-dimensional or two-dimensional nano pattern. With this feature, it is possible to control semiconductive and conductive properties and also possible to manufacture widespread functional devices including sensors, memories, etc., using various chemical bonds of carbon.
Despite excellent electrical, mechanical, and chemical characteristics of graphene described above, a research of a practically applicable technology has been limited since a mass production method of graphene has not been developed. In a conventional mass production method, graphite is ground mechanically and dispersed in a solution and a thin film is formed by means of a self-assembly phenomenon. Although graphene can be produced at a relatively low cost by the conventional method, electrical and mechanical characteristics cannot meet the expectations due to a graphene structure in which numerous graphene pieces are overlapped and connected with each other.
Due to a recent surge in demand for flat panel displays, a global transparent electrode market is expected to grow to about twenty trillion won within about 10 years. With development of a display industry in Korea, a domestic demand for transparent electrodes reaches hundreds of billions of wons every year. However, due to a lack of source technologies, Korea depends heavily on imports for transparent electrodes. An ITO (Indium Tin Oxide) as a representative transparent electrode is widely applied to a display, a touch screen, a solar cell, and the like. However, recently, a lack of indium has contributed to an increase in cost, and, thus, there has been an urgent need to develop a substitute substance. Further, due to fragility of the ITO, there has been a limit in applications of the ITO to next-generation electronic devices which is foldable, bendable, and extendable. On the contrary, graphene has been expected to have excellent elasticity, flexibility, and transparency and also expected to be produced and patterned by a relatively simple method. It is anticipated that a graphene electrode has a great import substitution effect if a mass production technology thereof can be established hereafter and also has an innovative ripple effect on the whole technologies in a next-generation flexible electronic industry.
However, due to lack of a method for effective synthesis, transfer, and doping, quality and a scale required to actually produce a graphene film have been restricted. By way of example, a conventional transparent electrode, such as an ITO, generally used for a solar cell exhibits unlimited scalability, optical transparency of about 90%, and sheet resistance of less than about 100 Ohm/square, whereas, the graphene film still shows the highest records of sheet resistance of about 500 Ohm/square, transparency of up to about 90%, and a scale of several centimeters. Korean Patent Laid-open Publication No. 2009-0026568 describes a method in which a polymer is coated on a graphitization catalyst and heat-processed to polymerize graphene, but this method needs to be performed at a high temperature of about 500° C. or more. Therefore, there has been a demand for development of a technique for readily producing graphene at a low temperature.